1. Technical Field
The present invention relates to a molecular pump and a flange and, more particularly, to a turbo molecular pump used, for example, for the evacuation of a vacuum vessel and a flange thereof.
2. Background Art
A molecular pump (vacuum pump) such as a turbo molecular pump and a thread groove pump has been often used, for example, for the evacuation of semiconductor manufacturing equipment or a vacuum vessel requiring a high vacuum for an electron microscope.
A suction port of the molecular pump is provided with a flange, and the flange can be fixed to an exhaust port of the vacuum vessel with bolts and the like. Between the flange and the exhaust port of the vacuum vessel, an O-ring, a gasket, or the like is provided to maintain the gastightness between the molecular pump and the vacuum vessel.
In the molecular pump, there are provided a rotor portion that is pivotally supported so as to be capable of being rotated at a high speed by a motor section and a stator portion that is fixed to a casing of the molecular pump.
For the molecular pump, the rotor portion and the stator portion accomplish an evacuating action due to the high-speed rotation of the rotor portion. By this evacuating action, gas is sucked through the suction port of molecular pump and is exhausted through an exhaust port.
Usually, the molecular pump exhausts gas in the molecular flow region (a region in which the degree of vacuum is high, and the frequency of collision between molecules is low). In order to demonstrate the evacuation capability in the molecular flow region, the rotor portion must be rotated at a high speed of, for example, about 30,000 revolutions per minute.
Usually, the molecular pump exhausts gas in the molecular flow region (a region in which the degree of vacuum is high, and the frequency of collision between molecules is low). In order to demonstrate the evacuation capability in the molecular flow region, the rotor portion must be rotated at a high speed of, for example, about 30,000 revolutions per minute.
In the case where some trouble occurs during the operation of the molecular pump, and the rotor portion collides with the stator portion or another fixed member in the molecular pump, the angular momentum of the rotor portion is transmitted to the stator portion or the fixed member, by which a large torque that rotates the whole of the molecular pump in the rotation direction of rotor portion is generated in a moment. This torque develops a high stress in the vacuum vessel via the flange.
A technique for easing a shock caused by such a torque has been proposed in Japanese Patent Laid-Open No. 2004-162696.
Japanese Patent Laid-Open No. 2004-162696 has proposed a technique in which a shock absorbing portion for absorbing a shock caused by the rotation torque of rotor is provided on a flange provided at the suction port end of the molecular pump.
Specifically, the flange is provided with a cavity portion adjacent to a bolt hole, and a thin-wall portion is formed between the bolt hole and the cavity portion. In the case where a shock in the rotation direction of a rotor portion is produced in the molecular pump, for example, by the fracture of the rotor portion, a bolt that fixes the flange of the molecular pump to a vacuum device hits the thin-wall portion, whereby the thin-wall portion is subjected to plastic deformation. By this plastic deformation of the thin-wall portion, the shock produced in the molecular pump can be eased.